1. Field of the Invention
This invention relates to a material useful as a hydrogel, i.e., a macromolecular material swollen with water. More particularly, the invention relates to a self-reinforcing hydrogel material having a high mechanical strength.
2. Description of the Prior Art
The term "hydrogel" as used herein indicates a macromolecular material having absorbed water therein, wherein at the equilibrium of swelling, the water absorption is at least 10%, preferably at least 20%, calculated according to the following equation: ##EQU1##
It is well known to prepare insoluble hydrogels by copolymerizing hydrophilic monomers such as glycol acrylate, glycol methacrylate, methacrylamide, acrylamide and the like, with cross-linking agents which are soluble in the monomer mixture (See, e.g., U.S. Pat. Nos. 2,976,576 and 3,220,960). It is also known to prepare insoluble hydrogels by copolymerizing vinyl ester monomers with cross-linking agents which are soluble in the monomer mixture and saponifying the resultant copolymer. It is moreover known that because of their hydrophilic characteristics, such materials are very valuable as hydrogel materials. Particularly, hydrogels exhibit good compatibility with the human body without stimulating the tissue or mucosa thereof, and they have attracted attention as a medical material. For example, hydrogels have been used for manufacturing contact lenses, various prostheses, pessars, implants containing biologically active substances, coatings of artificial dentures, coatings of surgical catheters, dialysis membranes for blood and other articles which are used in contact with the tissue or mucous membranes of the human body, etc.
Moreover, hydrogels are useful in other fields and for other purposes, such as for sizing synthetic fibers, for manufacturing membranes for dialysis and ultra-filtration, for chromatographic gels, etc. However, conventional hydrogels derived from polymers such as mentioned above are generally poor in mechanical strength and this defect has been a great obstacle to actual utilization of such hydrogels in the wet state, and in addition, they generally possess insufficient anti-thrombogenic properties.
Much research has been directed toward solving these problems of hydrogels of the above type, but the results have generally been unsatisfactory. For example, it is known that the mechanical strength of such hydrogels may be improved by copolymerization with diesters, by adding inorganic fillers thereto, etc. However, hydrogels thus obtained are not satisfactory, particularly in the wet state.